The present invention has for an objective an electrolytic material in the form of a solid gel for the modulation of light, in particular for the variable reflection of light and for the variable transmission of light. It also concerns electrooptical devices that use thin material, in particular very large devices such as windows and display cases of buildings, and windshields, windows, and roofs of automobiles. Self-evidently, it also concerns smaller devices such as screens, shutters, variably reflecting mirrors, display devices, and others that use the aforesaid material.
More precisely, the invention concerns an electrolytic material for the modulation of light of the type assuring the light modulation effect by the reversible electro-deposition of a metallic layer on the surface of a transparent electrode from metallic ions contained in the material. Such materials are also called electrochromic materials.
Devices for the modulation of light are already known consisting of two substrates of glass coated with a transparent, electrically conducting layer having a base of tin oxide or a mixture of indium oxide and tin oxide constituting two transparent electrodes between which is interposed a thin layer of electrochromic material, the assembly then constituting an electrolytic cell.
Polarization of a transparent cell of the type described above at low voltage causes the development of an absorbing and/or reflecting metal layer which is deposited on the negative electrode (cathode) by the reduction of metal ions present in the electrolytic medium. Simultaneously, an electrochemical oxidation reaction is produced by the positive electrode (anode). These two reactions must be electrochemically or chemically reversible in order to permit the return of the cell to the state of maximum transmission, either by imposing a voltage that is the reverse of the darkening voltage or by short circuiting the two electrodes, or finally, by a spontaneous return to the clear state in an open circuit. For certain applications such as, for example, windows and variably reflecting mirrors, it is desirable to maintain the cell in a given state of transmission and/or reflection lasting for a variable period reaching up to several hours before reestablishing the state of maximum transmission.
Examples of devices of the above type utilized for display signs of small surface area and primarily using silver as the metal constituting the optically active layer are specifically described in French patents or French patent applications No. 2,260,167 of Dec. 23, 1974 [U.S. Pat. No. 4,013,343 (Jaccard et al.)], No. 2,352,361 of May 19, 1976 [U.S. Pat. No. 4,153,345 (Duchene et al.)], No. 2,468,179 of Oct. 17, 1979 [U.S. Pat. No. 4,361,386 (Meyer)], No. 2,618,571 of Jul. 24, 1987 [U.S. Pat. No. 5,056,899 and U.S. Pat. No. 5,074,648 (Warszawski)], and U.S. Pat. No. 3,245,313 (Zaromb) of Apr. 12, 1966 and U.S. Pat. No. 3,626,410 (deKoster) of Dec. 7, 1971. The prior art devices utilize aqueous electrolytic media. The addition of a strong mineral acid is necessary in such devices in order to prevent hydrolysis of the metal salts that enter into the composition. The media formed in this manner exhibit low pH values and have the drawback of generating hydrogen gas concurrently with electrodeposition of the metal layer and/or of causing the degradation of transparent, electrically conductive material. In addition, organic electrolytic materials have also been proposed in certain prior art devices, especially those based upon methyl alcohol (French Patent No. 2,352,361) or a mixture of methyl alcohol and acetonitrile (French Patent No. 2,468,179) which demonstrate the drawback of mediocre stability which limits the range of their application because of the low boiling temperatures of proposed solvents.
Electrochromic materials free from these drawbacks would therefore be welcomed.
Moreover, consideration relating to safety and to the mechanical effects of hydrostatic pressure on the sealing properties demanded of very large devices that use electro-chromic materials make it desirable to form a deposit of an electrochromic material based upon the reversible electro-deposition of a metal layer present in solid form.